1. Field of the Invention
The present invention relates to a memory module comprising a printed circuit board formed by printing voltage terminals, ground terminals and various signal terminals (data terminals, control signal input terminals, address signal input terminals, etc.) on a substrate, the terminals being electrically connected to an external mother printed circuit board, and a plurality of memory devices (DRAM, SRAM, etc.) mounted on the printed circuit board.
2. Description of the Related Art
An example of a memory module is disclosed in U.S. Pat. No. 4,656,605, of which the entire disclosure is incorporated herein by reference.
An OKI Data Book entitled "MEMORY MODULE (FIRST EDITION)" issued in July, 1990 (of which the entire disclosure is incorporated herein by reference) also describes various kinds of memory modules. There have been known, for example, a 1 Mega-word by 9 bit DRAM module (Model No. MSC2312A manufactured by Oki Electric Industry Co., Ltd.) having a printed circuit board with nine 1 Mb DRAMs mounted thereon, a 1 Mega-word by 9 bit DRAM module (Model No. MSC2359 manufactured by Oki Electric Industry Co., Ltd.) referred to as a "three chip pack", which has a printed circuit board with two 4 Mb DRAMs and a 1 Mb DRAM mounted thereon, and a 256 Kiloword by 36 bit DRAM module (Model No. MSM2320A manufactured by Oki Electric Industry Co., Ltd.) called a "multi-nine", which has eight 1 Mb DRAMs and four 256 Kb DRAMs mounted thereon.
The mounting of all kinds of memory devices (such as those different in maximum access time from each other, or those whose makers are different from one another) on these memory modules is not necessarily allowed even if these memory modules are identical in capacity to each other. It is desirable to mount memory devices of Model Nos. compatible in temperament or characteristics, preferably, memory devices identical in Model No. to each other. However, memory devices which are different in Model No. from each other are needed in some memory modules, including 4MDRAMs and a 1MDRAM in a "three chip pack" and 1MDRAMs and 256KDRAMs in a "multi-nine". It has thus been desired from the standpoint of memory module design to enable memory devices to be combined in a memory module without considering whether or not the memory devices are compatible in temperament.